1. Field of the Invention
The present invention relates to disk drives and, in particular, to switching between servo and data modes in the read channel of a disk drive.
2. Background Art
Computers often use disk drives to store information. Disk drives typically include one or more circular disks having at least one ferromagnetic surface. A read and write head assembly is positioned in proximity to the surfaces of the disk. A read/write head receives information to be stored from the computer and alters the magnetic orientation of a portion of the ferromagnetic surface. The read/write head detects the orientation of the portion of the ferromagnetic surface and provides an output signal to the computer. Thus, information from the computer may be stored to disk, and information from the disk may be retrieved and provided to the computer.
To allow recovery of data from a disk, the head assembly must be precisely positioned over the portion of the surface of the disk where the desired data is located. Several methods exist for controlling head position. One method is referred to as dedicated servo. One disk surface is dedicated as a servo surface and another disk surface is dedicated as a data surface. Head positioning control information is recorded on the servo surface, while user data is recorded on the data surface. This method has several disadvantages. First, this method requires multiple disk surfaces (usually multiple disks), which results in a large disk drive structure that can be too cumbersome for many applications. Second, this method is intolerant of any misalignment between the heads associated with the servo disk surface and the heads associated with the data disk surface.
FIG. 3 illustrates another method, referred to as sector servo, which involves the placement of servo information and user data on the same disk surface. The surface of the disk is divided into regions for storing the servo information and regions for storing the user data. Regions S.sub.1 -S.sub.8 are used for storing servo information. Regions D.sub.1 -D.sub.8 are used for storing user data. The servo information and user data often have different recording characteristics. Thus, the read circuitry usually must adjust from reading servo information to reading a user data and from reading user data to reading servo information. This adjustment takes time. Since the disk is rotating past head 301, which is mounted on actuator arm 302, during the transition time, some angular amount of disk space will pass under the head assembly during the transition. Examples of the disk space passed during mode transition are shown by angular gaps 303 and 304 of FIG. 3. The disk is neither fully in data mode nor fully in servo mode during this time, so the disk space crossed during the transition may not be used for storing either servo information nor user data. Thus, some of the disk space is unusable for data storage.
Since the disk is rotating past head 301, the signals derived from head 301 over time will be based on information read from the disk as it passes under head 301. FIG. 5A illustrates the signals derived from head 301. Signals from servo sector S.sub.1 are read during time period 501. Time period 502 represents a delay required for the prior art read channel to switch from servo mode to data mode. Signals from data sector D.sub.2 are read during time period 503. Time period 504 represents a delay required for the prior art read channel to switch from data mode to servo mode. Signals from servo sector S.sub.2 are read during time period 505. Time period 506 represents a delay required for the prior art read channel to switch from servo mode to data mode. Signals from data sector D.sub.3 are read during time period 507. Time period 508 represents a delay required for the prior art read channel to switch from data mode to servo mode.